Is a Concrete Base Beneath a Vinyl Pool Liner a Pool Shell?

NEC 2017 680.26(B) states that conductive pool shells shall be bonded. 680.26(B)(1) states that, "Vinyl liners...shall be considered to be nonconductive materials." Some pool construction methods that utilize nonconductive vinyl liners also utilize a (unreinforced) concrete base (or shell) between native soil and the vinyl liner material. Normally, this concrete base layer does not come in contact with pool water unless there is a tear in the vinyl liner or an installation defect. The question posed in a code review seminar is whether this concrete base layer is considered to be part of a pool shell, and if so, whether it is required to be incorporated into the equipotential system using a copper conductive grid as described in 680.26(B)(1)(b).

In 680.26(B)(1) bonding is specified to be to "conductive pool shells," and not to conductive "materials."There appears to be no specific definition of what constitutes a pool "shell," and it is not clear in 680.26(B)(1) whether vinyl "liners" referenced as a "material,"are also considered to be a "shell."

You are correct in that the Code Panel in charge of Article 680 (CMP-17) has not yet defined "pool shell" but uses the specific term, and requires that a conductive pool shell be bonded, as we already know. I would stagger a wild guess that CMP-17 would also not know how to answer your question. Public Inputs would be needed to define that term. Public Inputs for 2020 code are now closed.

The question that comes to my mind is - how often does the vinyl liner fail? If it is often, then good practice would dictate that the concrete base be incorporated into the bonding grid. If there is steel or even mesh in the concrete base anyway - then sure, bond it in. If I err, I always let it be on the side of caution. But that's me.